Emerging Corynebacterium diphtheriae Species Complex Infections, Réunion Island, France, 2015–2020

Clinical, epidemiologic, and microbiologic analyses revealed emergence of 26 cases of Corynebacterium diphtheriae species complex infections on Réunion Island, France, during 2015–2020. Isolates were genetically diverse, indicating circulation and local transmission of several diphtheria sublineages. Clinicians should remain aware of the risk for diphtheria and improve diagnostic methods and patient management.

Clinical, epidemiologic, and microbiologic analyses revealed emergence of 26 cases of Corynebacterium diphtheriae species complex infections on Réunion Island, France, during 2015-2020. Isolates were genetically diverse, indicating circulation and local transmission of several diphtheria sublineages. Clinicians should remain aware of the risk for diphtheria and improve diagnostic methods and patient management.
Réunion Island during 2015-2020 and identify possible links with cases on other islands in the region.

The Study
We included all cases of C. diphtheriae species complex infections reported to the regional health agency and recorded at Réunion Island University Hospital during 2015-2020. We analyzed medical records and extracted age, sex, country of residence, recent travel, contact with animals, socioeconomic status, and diphtheria vaccination status for each case. We performed antimicrobial susceptibility testing; identified co-infecting strains; and determined tox gene presence, diphtheria toxin production, and biovar and sequence type (ST). We sent each isolate to the National Reference Center for Corynebacteria of the diphtheriae Complex (Institut Pasteur, Paris, France) to confirm species identity through multiplex PCR and biotyping as previously described (8)(9)(10). We detected the tox gene by using conventional PCR or, since 2019, by using multiplex real-time PCR (10). We assessed toxin production by using a modified Elek test (11). We determined antimicrobial drug susceptibility by using disk diffusion or by determining MICs (E-test; bioMérieux, https://www.biomerieux.com), in accordance with CASFM/EUCAST2021 (https:// www.sfm-microbiologie.org/2021/04/23/casfmavril-2021-v1-0) recommendations for benzylpenicillin, amoxicillin, cefotaxime, clindamycin, rifampin, and ciprofloxacin. We genotyped each isolate by using multilocus sequence typing (MLST) (12).

Conclusions
We report increased prevalence of cutaneous C. diphtheriae species complex infections on Réunion Island during 2015-2020. Introduction of mass spectrometry analysis in hospital laboratories and increased clinician awareness might have led to increased case reporting. Our study confirms that C. diphtheriae species complex members are circulating and are likely underestimated in the southwest Indian Ocean (7,13). Moreover, we observed emergence of locally acquired cutaneous C. diphtheriae infections on Réunion Island since 2019. The number of imported cases in 2020 was probably limited because of the COVID-19 pandemic, which reduced travel. Indeed, all C. diphtheriae cases identified during 2015-2018 occurred in patients who had traveled from other islands in the Indian Ocean. In addition, cutaneous diphtheria appeared to be associated with poor socioeconomic living conditions, in which alcoholism, drug dependence, and homelessness are factors that increase risk for human-to-human transmission and virulence (14).
A total of 8 (30%) C. diphtheriae isolates were toxigenic and caused cutaneous infections. Nontoxigenic C. diphtheriae isolates (70%, n = 19) were obtained from cutaneous lesions, respiratory samples, and bone samples. Clinicians should be aware that nontoxigenic C. diphtheriae can potentially cause severe disease (1,14,15). Moreover, all isolates were co-infected with pyogenic bacteria, suggesting diphtheria infection should be considered under polymicrobial conditions.
MLST analysis identified 21 different STs; most were unrelated (>2 mistmatches) reflecting marked genetic diversity of isolates. ST88 was found in 4 patients living on Réunion Island who had not traveled recently, indicating probable local acquisition. ST88 had previously been reported only in patients from Mayotte. Therefore, our results show that multiple C. diphtheriae species complex clones are circulating in the southwest Indian Ocean (8). Both C. ulcerans strains belonged to ST339. The National Reference Center reported that ST339 is the predominant C. ulcerans ST found in animals in France. Although considerable ST diversity was revealed, whole-genome sequencing will be required to further evaluate circulating C. diphtheriae clones in this region.
In conclusion, we describe emergence of locally acquired C. diphtheriae species complex infections on Réunion Island during 2019-2020. Local clinicians and microbiologists should remain aware of this neglected infection; improvements should be made in diagnostic methods and management of infected patients, such as maintaining availability of diphtheria antitoxin.
The National Reference Center for Corynebacteria of the diphtheriae Complex is supported financially by Santé publique France (Saint-Maurice, France).